1. Field of the Invention
The invention relates to an organic electroluminescent element (may be appropriately referred to as an organic EL element or an element hereinafter) that can be effectively used for surface light sources such as a full color display, a backlight and an illumination light source, and light source arrays of such as printers.
2. Description of the Related Art
The organic EL element comprises a luminescent layer or a plurality of organic compound layer including the luminescent layer, and a pair of opposite electrodes with interposition of the organic compound layer. Electrons injected from a cathode and holes injected from an anode are recombined in the organic compound layer of the organic EL element, and a light is emitted from the element by taking advantage of light emission from excitons formed by recombination, and/or light emission from excitons of other molecules formed by energy transfer from the excitons formed by recombination.
Luminance and element efficiency of organic EL elements have been largely improved by forming a laminated structure having different functions in respective layers. For example, frequently used elements include a dual-layer laminated element having a hole transport layer and a layer that serves as both a luminescent layer and electron transport layer, a three-layer laminated element having a hole transport layer, a luminescent layer and an electron transport layer, and a four-layer laminated element comprising a hole transport layer, a luminescent layer, a hole blocking layer and an electron transport layer (for example, see Science, Vol. 267, No. 3, 1995, p 1332 1).
However, practical application of the organic EL element yet involves many problems to be solved. In particular, the largest problem is deterioration of the quality during continuous driving, or incidence and growth of non-luminescent or low luminance regions (so-called dark spots).
A method proposed for preventing deterioration of luminance during driving is to eliminate interfaces of the organic layer in the element by providing a mixed region of a hole transport material and an electron transport material (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-305085). Deterioration of luminance is prevented by this method by suppressing electric charges from accumulating at the interface during driving by eliminating the interface between the organic layers in the element. However, the holes leaking out of the mixed region may be injected into the electron transport material, or the electrons leaking out of the mixed region may be injected into the hole transport material at the interface of the region adjacent to the mixed region. Accordingly, it may be apprehended that deterioration of the hole transport material from an anionic state or deterioration of the electron transport material from a cationic state may be caused.
In another proposed method, a region for forming a concentration gradient is provided between a carrier transport region consisting of only the electron transport material and the mixed region (see, for example, JP-A No. 2002-313584). However, the charge leaking out of the mixed region recombines in the concentration gradient region, and luminous efficiency may be decreased or the charge transport material may show deterioration from the excited state.